Along with a reduction in physiological properties, the natural history of aging may include a mechanical plasticity emphasizing proximal muscles as a compensatory mechanism for reduced distal muscle function in locomotion. Pilot data show that while walking at the same speed, healthy old vs. young adults produced more torque and power with proximal hip muscles and less with distal knee and ankle muscles. Since old adults may have larger physiological deficits in distal vs. proximal muscles this mechanical plasticity in torques and powers may enable old adults to perform gait tasks with a motor control strategy favoring their physiological characteristics. We hypothesize that healthy human aging involves mechanical plasticity in locomotion that produces a distal to proximal shift in muscle function. Hypothesis will be tested with 4 specific aims: 1) demonstrate the existence of mechanical plasticity with age and test the effect of gait speed on mechanical plasticity in level walking; 2) determine the relationship between age and mechanical plasticity; 3) determine the generalizability of mechanical plasticity by comparing gait in young and old adults on stairs and an inclined walkway; and 4) assess the effect of strength on mechanical plasticity in locomotion. 140 adults approximately evenly distributed through the ages of 18-85 yrs will be screened for health and mobility status and measured with video, force plates, and EMG in level, stair, and incline walking. Lower extremity strength will be measured isokinetically. Inverse dynamics will calculate lower extremity joint torques and powers. Factorial ANOVAs and regression analyses will be used to analyze joint kinematics, torques and powers and their relationships with age. We expect greater hip and less knee and ankle torque and power in old vs. young. We expect curvilinear relationships between age and measures of mechanical plasticity (torques & powers). We expect that strength will have no effect on mechanical plasticity with all strong and weak old adults exhibiting the altered motor strategy. Our long term goal is to establish a theoretical framework of biomechanical adaptations with age in locomotion. This framework provides a basis for designing exercise and rehabilitation programs for aged individuals with mobility impairments. [unreadable] [unreadable]